(1) Field of the Invention
The present invention relates to a frequency stabilizing circuit used for a pulse modulator that modulates an image signal including a synchronous signal into a pulse modulation signal.
(2) Description of the Related Art
Recent development in the field of a fiber-optic transmission technology makes it possible to transmit an analog signal such as an image signal.
There are various modulation methods available for the image transmission such as analog, pulse, and digital modulations. However, a primary choice is pulse modulation that generally modulates an image signal into a pulse signal having amplitudes of 1 and 0, while modulating its pulse repetition cycle and/or its width analogously. Because it not only minimizes influence of non-linear distortion on transmission lines effectively, but also simplifies the construction of a modulator and a demodulator, PFM(Pulse Frequency Modulation) is most suitable for image signal transmission because its post-transmission noise is delta noise which is hardly noticeable. For this reason, PFM has been used widely in the image signal transmission via optical fibers these days.
FIG. 1 shows frequency characteristics of an output signal from a modulator for PFM. An output pulse signal's frequency f varies in proportion to an input image signal's voltage V, and its characteristics are determined by the capacity of a capacitor incorporated in the modulator or a modulator's response speed. These devices are susceptible to fluctuation in ambient temperature and in power source voltage; the frequency characteristics change in response to such fluctuations. For example, the frequency characteristics expressed as a solid line change to a dotted line or an alternating and short dot line due to temperature or voltage fluctuations. In other words, a frequency f1 corresponding to an input voltage V1 changes to a frequency f2 or a frequency f3.
Frequency fluctuation is a critical problems to be resolved, because it not only deteriorates the linearity of the transmitted image signal, but also makes frequency characteristics unstable. A relatively easy method to minimize the frequency fluctuation is to employ an element or a circuit that maintains reverse characteristics to those of the capacitor so as to compensate the operation of the modulator. However, when its accuracy is concerned, such a compensation is effective only in a limited operation range and for a limited period.
As for a method to stabilize the frequency for a longer period, an automatic frequency stabilizing circuit is disclosed in Japanese Laid-Open Patent Application No. 2-5050. According to this method, the frequency of an output signal from the modulator for PFM is detected to be compared with that of a standard signal from another circuit incorporated in the circuit, and the difference therebetween is fed back to the input unit of the modulator.
For further explanation, the detailed construction of such an automatic frequency stabilizing circuit is explained with referring to FIG. 2.
The frequency stabilizing circuit is composed of a modulator 1 for PFM, a clamping circuit 2, an adder 3, a synchronous signal sampling circuit 4, a duty compensating circuits 5 and 7, a standard frequency oscillator 6, a digital phase comparator 8, a switch unit 9 a charge pumping circuit 10 including switches 10a and 10b as well as a capacitor 11, and a lowpass filter 12.
An image signal including a synchronous signal is inputted to the modulator 1 via the clamping circuit 2 and adder 3. Upon the receipt of the image signal, the modulator 1 modulates it into a pulse signal with[a frequency fa, and outputs it as an output PFM signal to the outside apparatus as well as to the duty compensating circuit 5. The standard frequency oscillator 6 operating independently to the modulator 1 outputs a signal with a specific frequency fb to the duty compensating circuit 7. Both the frequencies fa and fb are inputted into the digital phase comparator 8 from the respective duty compensating circuits, so that it compares these two frequencies. However, since the PFM signal's frequency fa varies due to the modulation, only a stable frequency fa corresponding to the synchronous signal must be sent to the capacitor 11. For this reason, the synchronous signal sampling circuit 4 detects the synchronous signal, and outputs a detection signal to the switch unit 9 as an enable signal.
When the frequency fa is lower than the frequency fb(fa&lt;fb), the switch 10a is turned on so as to charge the capacitor 11, whereby a voltage inputted to the lowpass filter 12 is increased. Accordingly, the output voltage from the lowpass filter 12 and hence the image signal's voltage inputted to the modulator 1 is increased by means of adder 3. Then, a frequency of the PFM signal outputted from the modulator 1 is also increased. The foregoing operation is repeated until the frequencies fa and fb become subsequently equal.
In contrast, when fa is higher than fb(fa&gt;fb), the switch 10b is turned on so as to discharge the capacitor 11, whereby a voltage inputted to the lowpass filter 12 is decreased. Accordingly, the output voltage from the lowpass filter 12 and hence the image signal's voltage inputted to the modulator 1 is decreased by means of the adder 3. Then, a frequency of the PFM signal outputted from the modulator 1 is also decreased. The foregoing operation is repeated until he frequencies fa and fb become subsequently equal.
In summary, the conventional frequency stabilizing circuit compares the two frequencies of the output signals, one from the modulator 1 and the other from the standard frequency oscillator 6, using the digital phase comparator 8, and stabilizes the frequency by either charging or discharging the capacitor 11. However, given that the PFM signal outputted from the modulator 1 has a very high frequency, devices such as digital phase comparator 8 and standard frequency oscillator 6 must be constructed as such, which invites a high-speed and large-scale signal processing circuits that require a large amount of running electricity as well as manufacturing cost. In addition, the conventional frequency stabilizing circuit can not resolve the problem of noise due to its construction that includes essentially two signal generators: the modulator 1 and standard frequency oscillator 6. Moreover, it should be noted that such noise may further cause an error thereof.